The Federal Communication Commission (FCC) has recently set aside spectrum for the Narrowband Personal Communications Services (PCS) channels. The devices that will operate in the Narrowband PCS channels must adhere to a one part-per-million (ppm) frequency stability requirement for their transmitters. For example a transmitter operating at 940 MHz must be accurate in center frequency to within 940 Hz to satisfy the 1 ppm stability requirement. This spectrum may be used for many services, including advanced messaging and paging, two-way pagers, and acknowledge-back (ack-back) pagers. Ack-back pagers are those selective call receivers that not only receive but also transmit (automatically and/or manually) an acknowledge signal in response to receiving their selective call address or a message, such as that described in U.S. Pat. No. 4,891,637 to Siwiak et al. assigned to the assignee of the present application, incorporated herein by reference and described below.
The components of an ack-back pager, for instance, include an antenna coupled through a transmit/receive switch to either a transmitter or a receiver. A second antenna may also be utilized, thereby eliminating the need for a switch. The general operation of the pager is elaborated below.
In current paging systems, paging signals are transmitted from a paging transmitter to a multiplicity of portable paging receivers according to a prespecified modulation format including, for example, serialized digitally coded sync, address, and message data words. Each paging receiver includes an input stage which receives, demodulates, and converts the paging signal into an analog signal having a DC bias level.
The receiver circuits usually comprise a combination of filters, mixers and oscillators. The receiver circuits generate a recovered signal suitable for processing by a discriminator. Typically the bandwidth of the intermediate frequency (IF) filter(s) is small compared to the high frequency received signal. Therefore, to maintain proper operation, the requirements of the first local oscillator become extreme in terms of frequency stability.
In the next stage of the paging receiver, amplitude signals are generated by a data limiter or microprocessor. For example, the amplitude signal could be representative of binary states (e.g., zero or one), for the duration over which the amplitude of the analog signal is above or below a set threshold level. The serially generated binary states represent bits of the digitally coded words of the paging signal. There could also be a multiplicity of states that would correspond to a greater number of bits per state.
The DC level is not considered fixed, but rather relative to the recovered paging signal. Consequently, the threshold level of the data limiter is not set at a constant value, but rather is dynamically acquired from the immediate paging signal.
Most paging transmission formats include a digitally coded word (e.g., sync word) to synchronize the operation of the various paging receivers to the digitally coded address words transmitted successfully thereafter in a transmission cycle. Paging receivers are customarily preprogrammed with the slot of the sync word and the slot in which the corresponding address is expected to be transmitted with respect to the sync word.
After receiving a message, some pagers allow for acknowledge-back data to be transmitted. The user may input the data into the microprocessor by a variety of means. The microprocessor processes this data and supplies binary output data from output port to the input port of a digital to analog (D/A) converter. The output signal is passed to the transmitter for processing and transmission. The transmitter generally includes frequency synthesis circuitry. Such circuitry may include a crystal oscillator, a varactor, a voltage controlled oscillator, RF amplifiers and filters.
In order to maintain proper operation, since the requirements of the first local oscillator become extreme in terms of frequency stability, a high stability crystal could be required. While such high stability crystals are obtainable, the prices are often prohibitively high and require temperature compensation circuits to control the stability of the total oscillator circuit. Furthermore, to operate in the Narrowband PCS channels the transmitter must adhere to a one part-per-million (ppm) frequency stability.
An alternative to using a high stability reference crystal is an Automatic Frequency Control (AFC) scheme. This would allow the portable units to use the base station transmitters as an accurate frequency reference and make adjustments according to its received signal.
However, in land mobile environments, a sufficiently strong signal that could be used as a reference does not always exist. In addition, channel conditions such as Rayleigh fading and Doppler shifting could make it impossible for the portable unit to discern the exact frequency of the transmissions. The portable unit must recognize when it is receiving a sufficiently strong and clear signal.
An AFC scheme could require the forward-channel transmitters to output an unmodulated, continuous-wave signal for a given time in order that the paging receiver can lock on to this signal. The output of this signal from the demodulator would be one constant voltage level that would correspond to the intermediate frequency to which the pager is tuned. The error in this measured voltage within the discriminator is mainly due to the frequency error of the local oscillators within the pager. Based on this error information, the pager can correct the frequency of its local oscillator. Drawbacks to such an AFC scheme involve use of the carrier's air time to provide an unmodulated signal which is prohibitive in cost and limits the band's capacity.
Another method of AFC used in two-way paging schemes involves the detection of the paging signal and then mixing the signal down to an intermediate frequency (IF) with local oscillators within the pager. The discriminator then detects the frequency shifts at the IF that transmit the data in such a Frequency Shift Keying (FSK) system. The highest and lowest (peak and valley) shifts are measured within the discriminator. The average of the two (audio median) would be the voltage level associated with the intermediate frequency to which the pager is tuned. A demodulated reference is provided which is representative of an output of the demodulator when the input is a zero frequency error intermediate frequency. The error between this audio median and the demodulator center in this measured voltage within the discriminator is mainly due to the frequency error of the local oscillators within the pager. Based on this error information, the pager can correct the frequency of its local oscillator.
However, one problem with this error detection scheme is that the error information on the received peaks and valleys determined in the discriminator could likely be based on artificial peaks or valleys. These artificial peaks or valleys may be due to channel conditions such as Doppler shifts, Rayleigh fading, or spurious signals The artificial peaks or valleys would result in an incorrect frequency error determination. Accordingly, the accuracy of this AFC scheme is limited.
Thus, there is a need in the art for an economical AFC scheme that increases the overall stability of a selective call receiver/transmitter without the requirement of a high stability reference crystal oscillator.
Furthermore, there is a need in the art for a method and apparatus for determining received signal strength to detect signal conditions during AFC calculations and correct the crystal frequency only when AFC information is accurate.